Groundwater is the sole source of potable water for the residents of the Indian Wells Valley Groundwater Basin. Basin-wide pumping of groundwater is estimated to be approximately 27,000 acre-feet per year (one acre-foot = 325,851 gallons).
Recharge to the groundwater system is estimated to be approximately 10,000 acre-feet per year. Because the quantity of water coming out of the system is approximately three times the amount coming back into the system, the Basin is determined to be in overdraft. As the groundwater supply is a finite and limited resource, it behooves those dependent on groundwater to acquire as much data as possible to better understand the systemâ€™s longevity and determine best management practices for its use.
Various government entities have been measuring depth to groundwater in the water wells of the Basin since the 1920s. These monitoring programs were first initiated by the United States Geological Survey (USGS) and were handed over to the Kern County Water Agency (Agency) in 1989. The Indian Wells Valley Water District, United States Bureau of Reclamation (USBR) and Naval Air Weapons Station, China Lake (NAWS) have also assisted in the collection of groundwater level data within the basin. Additionally, these entities and basin residents have been instrumental in obtaining access to production water wells for collection of data as well as construction of various wells dedicated solely to monitoring groundwater levels.
Since 1989, the Agency has been measuring depth to groundwater in an ever-expanding network of private and public water production wells and monitoring wells. Water level readings are taken twice per year in October and March. The reasoning behind the two measuring times is that historically, groundwater demand reaches its peak by October and demand will be at its minimum by March. The water table of the groundwater system lowers when pumping increases and rebounds when pumping decreases. Knowing the highest and lowest levels of groundwater in the aquifer system is crucial for water well design and determining the proper depth for setting a pump. This is the ideal condition, but, in reality groundwater demands vary for many reasons.
Agriculture usually sees its maximum demand by October and its minimum demand by March, depending upon the crop type. Some crops are harvested year round, so this can result in a relatively stable demand throughout the year with variations dependent on temperature. Urban water demand has many components or cycles.
There are daily demand maximums which happen in the morning 6-8 a.m., and in the evenings 5-7 p.m. There are also weekday versus weekend, and seasonal demand variations. In spite of the many cycles of groundwater level variations, the October and March readings give the best overall picture of the dynamics of the aquifer system.
There are numerous devices used to measure groundwater levels. The simplest is the steel surveyorâ€™s tape, the tip of which is lowered to a depth that is known to be below the groundwater table. The amount of tape sent down the well is recorded at some known reference point, this is usually the vent tube at the well head (for example, 400 feet of tape went down the well). The tape is pulled out and the distance on where the tape is wetted is then recorded (eg. 100 feet). The distance between the two measurements is 300 feet which is the distance from the reference point to the groundwater surface. The height of the reference point above ground surface (eg. 2 feet) is measured and subtracted from this value, thus the distance from the ground surface to groundwater level surface, commonly referred to as â€œdepth to groundwaterâ€ would be 298 feet. It is also valuable to know the elevation of the groundwater surface based on mean sea level. This can only be calculated by first determining the elevation of the ground surface and the elevation of the reference point. These values can be determined from USGS topographic maps, surveying from a known reference or Global Positioning Satellites (GPS).
Other methods for measuring this distance to groundwater in a well are the plopper, electrical sounder, airline, acoustic devices and digital transducers. A plopper is a 1 inch galvanized nipple attached to the end of a cloth surveyorâ€™s tape which is lowered down the well until it hits the groundwater surface. The result is an audible â€œplopâ€. At that point, a direct reading is taken. Ploppers are by far the quickest, easiest and most accurate method for measurement. Their drawback is that oftentimes there is not a large enough access hole to get the nipple into, or pumps and wiring in the well obstruct the device.
An electrical sounder is either a single wire with a weight at the end that has been marked in ten foot increments which is connected to a battery and some type of meter or audible device and then grounded to the well. When the end of the wire hits the water surface, an electrical circuit is completed and registered on the meter and/or audible device. A dual wire electrical sounder is nothing more than both wires going down the well at the same time, completing the circuit when they hit the groundwater table. Electrical wire sounders, if properly marked, can be very close to the same accuracy as the surveyorâ€™s tape and the plopper which canapproach plus or minus one tenth of a foot.
Airline sounders are usually a permanently placed, small diameter tube of plastic or steel of known length which runs from the well head to somewhere below what is expected to be the lowest level in the well that the water table is expected to reach. It is then connected to a pressurized gas source (compressed nitrogen is themost common) and a pressure gauge. The gas valve is opened and the pressure increased in the line until it stabilizes. This is the amount of pressure needed to displace the water in the air line. Depending on the length of tubing and the pressure needed to displace the water, a calculation is then made that generates the distance
from the well head to the water table. Airline measurements are notoriously inaccurate and usually can only be trusted to plus or minus 17 feet.
An acoustic sounder is a device that sends out a sound wave and then calculates the travel time between when it leaves the device, hits the water surface and bounces back to the device. Early models could be very inaccurate at plus or minus 20 feet due to changes in air density in the well caused by temperature, pumps and wiring, and roughness of the well casing. Newer versions of these devices are reported to give accuracy on par with electrical wire sounders. Digital transducers are made up of a pressure sensitive crystal wired to a small computer with memory. It is lowered into the well by a data transmission cable. Transducers can be programmed to take a reading with intervals as low as every second to once per year. Accuracy for transducers can approach plus or minus one tenth of a foot.
A future article will discuss processing of the field data, data archival, differences between monitoring wells and production wells and interpretation of the data. Additionally, there will be discussion concerning what makes an effective groundwater monitoring program, filling of data gaps and data accessibility